Vector control of a permanent magnet synchronous motor (PMSM) requires accurate information of the rotor position. The simplest method to determine the required rotor position is to use a mechanical sensor. However, the sensor coupled to the shaft is a mechanical component liable to faults, which increases the price of a motor drive considerably. In addition, as the motor drive control is based on information obtained from the sensor, sensor breakage inevitably causes downtime in said motor drive.
In sensorless control, the rotor speed and position are estimated without mechanical sensors either by a fundamental-excitation method or a high-frequency (HF) signal injection method. Fundamental-excitation methods are based on the dynamic model of the motor, while signal injection methods are based on detecting the anisotropy caused by the saliency of the rotor or by magnetic saturation.
The most common ways in connection with the permanent magnet synchronous machine are based on using various flux observers. The operation of the flux observer is based on a voltage model on a synchronous machine, the model being based on the voltage equation of the machine. The initial values required by the voltage equation are inductances and resistances of the machine.
The voltage models produce an accurate estimate on the angular speed of the rotor at its higher values. However, the voltage models have a drawback that at near-zero speed the estimate obtained by the voltage model becomes inaccurate, due to possibly erroneous parameters and measurement inaccuracies together with a low back EMF produced by the machine.
In signal injection methods an extra signal deviating from the fundamental frequency is injected to the motor either in voltage or in current form. This signal provides in the machine an injection frequency response, from which is obtained by demodulation a position tracking signal that can be used for determining the position of the rotor.
Fundamental-excitation methods, such as methods based on flux or voltage models, have good dynamic properties, but they do not allow sustained operation at low speeds. On the other hand, signal injection methods are well suited to operation at low speeds, including standstill, but tend to have limited dynamic performance. For PMSM drives, approaches for combining a fundamental excitation method and a signal injection method have been presented in references [1]–[4].
The problem associated with the prior art methods for determining the position angle is the inaccuracy especially in low-speed region and in zero speed due to possibly erroneous parameter values and sensitivity to disturbances.